(1) Field of the Invention
The present invention relates to a paint for a metal can comprising an epoxy resin component and a curing agent component, which is highly improved in the property of retaining a high adhesion even with the lapse of time.
(2) Description of the Prior Art
As cans for canned provisions and canned drinks, there have ordinarily been used so-called three-piece cans comprising a can body having on the side face a seam formed by soldering, bonding or welding and top and bottom lids double-seamed to the can body, and so-called two-piece cans comprising a seamless can body formed by drawing or draw-ironing and a lid double-seamed to the can body. In each of these cans, a protective coating is formed on the inner face in order to inhibit the metal from being dissolved out into the content and prevent corrosion of the can.
As the paint for forming such protective coating, there have been broadly used mixtures or precondensates of an epoxy resin component with other thermosetting resin component such as a resol type phenolic resin or an amino resin. In manufacturing cans, such paint is coated on a can blank before or after formation of a can body and the coated paint is baked to form a thermally cured coating. Coatings formed from conventional paints for cans are excellent in the adhesion to can blanks just after baking, but the adhesion is degraded with the lapse of time and this degradation of the adhesion with the lapse of time is further accelerated by retort sterilization or by contact with the aqueous content or its vapor.
This degradation of the adhesion of the protective coating with the lapse of time is not preferred from the viewpoints of prevention of corrosion of the can blank and retention of the flavor of the content, and furthermore, this degradation results in serious reduction of other properties of the can.
For example, in case of an easy-open lid which can easily be opened by a purchaser without using any particular tool, reduction of the adhesion of the coating causes a phenomenon in which the coating is left in the opening of the can in the form of a film or feather ordinarily called "enamel feather". More specifically, opening of the easy-open lid is performed by outwardly pulling an opening portion defined by a score (incompletely cut line) through a tab secured to this opening portion to break the score and taking out the opening portion. In this case, if the bonding force of the coating to the substrate is smaller than the tear strength of the coating per se, the coating is not correctly broken along the score and the coating present in the opening portion is left on the side of the can lid to form an enamel feather. Since it is difficult to prevent degradation of the adhesion with the lapse of time in thermally cured coatings, occurrence of this enamel feather has heretofore been prevented by reducing the tear strength by decreasing the thickness of the coating. However, reduction of the thickness of the coating should naturally render it difficult to attain complete cover of the can blank, especially complete cover of the score-formed portion, and the effects of inhibiting dissolution of the metal and preventing corrosion become insufficient.
Furthermore, in bonded cans where the side seam is formed by bonding, reduction of the adhesion of the coating causes leakage from the side seam with the lapse of time. This side seam is formed by lapping both the edges of the can blank to be bonded and heat-bonding the lapped edges through the thermally cured coating on the can blank by a polyamide adhesive. In this bonded can, the adhesion between this coating of the conventional epoxy resin type and the can blank is degraded with the lapse of time and leakage is finally caused.